The invention relates to a charge neutralizer, and more particularly, to such an apparatus including a charge neutralizing brush formed by thin, conductive wires and which is brought close to a charged body to cause a discharge thereof.
It is known to remove an electrostatic charge from a paper, cloth, plastic film or the like by using a charge neutralizing brush formed by thin conductive wires or a charge neutralizing fabric which is woven from thin conductive wires. This technique is in practical use, and is based on the so-called corona discharge phenomenon wherein a thin conductive wire is placed in proximity to a highly charged body to establish a sufficiently high electric field around an edge of the wire causing an ionization of gas and producing a discharge of the charged body.
An example of such charge neutralizer will be described below with reference to an electrophotographic apparatus which employs it. In electrophotography, a record sheet to which the image of an original is copied may be highly charged as a result of various electrical operations which take place during the copying cycle such as a image transfer, deposition of charged developing toner thereon and/or the friction of the record papers with each other or with mechanical parts. Hence, if copies produced from the copying apparatus are in a charged condition, a number of inconveniences may result. For instance, interference with a proper alignment and stacking of record papers as a result of a repelling effect between similarly charged sheets disposed on a tray may occur or an operator may receive an electric shock or experience the hairs on his body being attracted when placed near such charged copies. A charge neutralizer is used to prevent such inconveniences.
Referring to FIG. 1, there is shown a schematic view of a portion of electrophotographic apparatus 1 and in particular, the record or copy delivery portion thereof. A fixing unit 2 is shown to the leftmost of this Figure, and includes a delivery outlet 2a, adjacent to which are paper of delivery rollers 3a, 3b. The paper delivery rollers 3a and 3b are disposed for rotation on rotary shafts 9a, 9b so as to abut against each other with record paper 6 interposed therebetween. Charge neutralizing brush 4, which forms charge neutralizer 5, is disposed after these rollers in the path of paper flow, and a tray 7 is provided below the brush for receiving record papers 6 therein. As shown, brush 4 is fixedly mounted on a stationary member indicated by numeral 8. Fixing unit 2 is of a conventional heater type in which a toner image is thermally fixed. After the fixing step, record paper 6 is driven to the right, as viewed in FIG. 1, into the nip formed between rotating delivery rollers 3a, 3b to be carried into tray 7. As record paper 6 passes the charge neutralizer 5, any electrostatic charge which is carried by the record sheet is discharged, thus achieving a charge neutralization.
FIGS. 2A to C illustrate conventional charge neutralizing brushes. Brush 4A shown in FIG. 2A comprises a number of thin metal wires 41 of high electrical conductivity which are distributed in an array having a uniform density, so that their free ends are aligned. The opposite ends of the thin wires are held between folded limbs of metal sheet 42 which may be formed of aluminium or the like. Brush 4B shown in FIG. 2B comprises subassemblies of thin conductive wires 43 which are arranged in the manner of a comb and which have one end held between folded limbs of metal sheet 44 which may also be formed of aluminium. Brush 4C shown in FIG. 2C comprises charge neutralizing fabric 46 using thin conductive wires 45 as part of its warp and which has its one end held between folded limbs of metal sheet 47, again formed of aluminium or the like, with thin wires 45 projecting forwardly of the fabric. The thin conductive wires may also be formed by metal plated fibres or metal coated fibres, in addition to the metal wires.
FIG. 3 graphically shows the charge neutralizing effect of charge neutralizer 5. As shown, the charge neutralizing effect is significant when the distance d between record sheet 6 and the front end of brush 4 is reduced. When the front end of brush 4 contacts record sheet 6, or when the distance d is reduced to 0 mm or less, the brush is most effective. It is considered that the charge neutralizing effect is at its maximum when brush 4 contacts record sheet 6 because a flow of charge from record sheet 6 occurs through brush 4, in addition to that associated with the corona discharge effect.
It is to be noted that the charge neutralizing effect of charge neutralizer 5 depends not only on the distance d between record sheet 6 and brush 4, but also on the degree of contamination of brush 4. As indicated in FIG. 4, when brush 4 is not contaminated, the charged potential of the record sheet can be reduced from a charged level A to a low level B which is close to zero. However, when brush 4 is contaminated, the residual potential after the charge eliminating step will reside at a level C which is substantially more than the B level. Thus, as contamination of brush 4 substantially degrades the charge neutralizing or eliminating effect, it is undesirable that the front end of brush 4 be allowed to contact a charged record sheet 6 so that contamination from contact is minimized. Therefore, it will be seen that a lasting charge neutralizing effect cannot be maintained under conditions which provide an optimum charge eliminating effect. Specifically, if brush 4 is allowed to contact record sheet 6 to enhance the charged elimiating effect, any unfixed toner which remains due to an imperfect heating or pressure fixing action may be deposited on brush 4. Similarly toner in its softened condition from heat fixing may attach to brush 4, before it is sufficiently cooled, thus causing a rapid loss of the neutralizing ability of brush 4.
In addition, a conventional charge neutralizer 5 is disposed at a fixed location, so that brush 4 cannot be maintained in sufficient proximity to record sheet 6 to achieve a stabilized charge neutralizing effect if record sheet 6 is not perfectly planar or its delivery path varies. Thus, referring to FIG. 1, it will be noted that at the time the leading end of record sheet 6 has left delivery rollers 3a, 3b, it will assume a position indicated by a solid line while it will assume another position indicated by phantom line 6A which is more spaced from brush 4 when a greater length of record sheet 6 has left the delivery rollers. Hence, if brush 4 is located at a position which is preferred for eliminating charge from the leading portion of record sheet 6, the same brush will be less effective in eliminating charge from the trailing part of record sheet 6. Conversely, if the position of brush 4 is chosen so as to be optimum for eliminating the charge from the trailing part of record sheet 6, the delivery path of record sheet 6 will be such that the leading portion thereof will bear against brush 4 to interfere with the delivery operation of record sheet 6. Thus, it has been impossible to locate brush 4 so as to achieve an optimum charge neutralizing effect for both the leading and the trailing portions of the record sheet.